This invention relates to a communications apparatus employing optical fibers, and more particularly to an optical communications apparatus having a repeater station.
FIGS. 8 and 9 are plan views of a prior-art optical communications apparatus, more particularly to optical-branching communications apparatus (hereinbelow, termed the "first prior-art example") such as are described in, for example, the official gazette of Japanese Patent Application Laid-open No. 73225/1987 entitled "Optical switch." FIG. 8 shows the state in which a prism is inserted in the optical paths, while FIG. 9 shows the state in which the prism shown in FIG. 8 has been taken out of one of the optical paths.
FIG. 10 is an arrangement plan showing the optical transmission system of the first prior-art example. Hereinbelow, the expression "communications node" shall signify a communications equipment or a communications equipment office, which will also termed as simply an office.
In FIGS. 8 and 9, numeral 1 designates an optical switch including a transmission prism 2 which is movable so that, when any office has broken down, the prism can be removed from the optical path for that office. The optical switch 1 also includes an upper light input terminal 3, a lower light input terminal 4, a lower light output terminal 5 and an upper light output terminal 6. The prior-art system in FIG. 10 includes Office-B 7, Office-C 8, Office-D 9, Office-A 10 and an optical fiber 11. Throughout the drawings, the same symbols denote identical or equivalent portions.
Now, the operation of the first prior-art example will be described with reference to FIGS. 8, 9 and 10.
Referring to FIGS. 8 thru 10, the transmission prism 2 delivers light from the upper light input terminal 3 to the lower light output terminal 5, from which the light proceeds to pass through, for example, the office-B 7 in FIG. 10, to pass through the lower light input terminal 4, to pass through the transmission prism 2 again and to pass through the upper light output terminal 6, whereupon the light is transmitted to the optical switch 1 of the next office, Office-C 8. In this manner, the light signal from the office-A 10 in FIG. 10 is transmitted toward the office-D 9 through the optical fiber 11. In the normal operation, the light signal is amplified in the respective offices of the office-B 7, office-C 8 and office-D 9 so as to compensate for attenuations in the optical paths 12. When trouble develops in any of the offices, the transmission prism 2 is moved and therefore is removed from the upper optical path 12 of the optical switch 1, as illustrated in FIG. 9, such that the light is transmitted to the next office without being branched and amplified.
Since the apparatus of the first prior-art example is constructed in the above manner, it has several of the following problems: The optical transmission prism 2 is expensive and is difficult of mass produce and is necessary for the optical branching. Moreover, the apparatus becomes high in cost and large in size because of the necessity of a driving mechanism (not shown) for mechanically taking the transmission prism 2 out of the main optical path 12, as illustrated in FIG. 9, when the light amplifier section of any office has broken down. Besides, it is required to establish a unidirectional loop ring which extends from the office-A 10 via the office-B 7 and office-C 8 to the office-D 9 and then to the office-A 10 again as illustrated in FIG. 10.
Next, an optical communications apparatus in the second prior-art example stated in the official gazette of Japanese Patent Application Laid-open No. 49526/1986 will be explained with reference to an arrangement plan shown in FIG. 11.
In FIG. 11, numeral 70 designates a change-over unit, which corresponds to the optical switch 1 including the prism 2 in the first prior-art example shown in FIG. 8. Numeral 71 indicates a monitoring control unit, and numeral 72 a regeneration unit for amplifying a signal. These elements 70-72 constitute a repeater 74, which corresponds to the office-B 7 including the optical switch 1 in the first prior-art example shown in FIG. 10. The repeater 74 has the change-over unit 70 operated by a change-over control signal from an adjacent office. An optical fiber 75 is laid on the side of Office-A, and an optical fiber 76 on the side of Office-C. These optical fibers 75 and 76 correspond to the optical fiber 11 in the prior-art example shown in FIG. 10.
The optical communications apparatus of the second prior-art example thus constructed operates as follows:
Usually, the switch of the change-over unit 70 assumes a state indicated by solid lines. Under this state, the received signal from the side of the office-A is applied to the input side of the regeneration unit 72 and is amplified by the regeneration unit 72, and the amplified signal is delivered to the side of the office-C. That is, the repeater 74 functions as a downward circuit on this occasion.
The monitoring control unit 71 is normally monitoring the output signal of the regeneration unit 72. When the change-over control signal has been sent from the side of the office-A, the monitoring control unit 71 detects this change-over control signal and issues a change-over command to the change-over unit 70. The switch state of the change-over unit 70 is transferred into a state indicated by broken lines, by the change-over command. Then, the received signal from the side of the office-C is applied to the input side of the regeneration unit 72 and is amplified by the regeneration unit 72, and the amplified signal is delivered to the side of the office-A. On this occasion, the repeater 74 functions as an upward circuit.
Accordingly, the circuit is utilized for bidirectional communications as the upward and downward ones, depending upon the change-over control signals from the adjacent offices.
As stated before, the first prior-art example has had problems in that the optical transmission prism, which is expensive and difficult of mass production, is used in the construction of the offices, that the offices need to be connected by the unidirectional loop ring, and that the driving mechanism for moving the prism in case of the breakdown of any office is required, so the apparatus becomes high in cost and large in size.
Also the second prior-art example has had problems in that the change-over control signal of the optical fiber must be sent from the other office, so the procedure of communications becomes complicated, that the monitoring control unit for the change-over control signal is required in the own office, so the circuit arrangement becomes complicated, and that a driving mechanism for mechanically changing-over the optical fibers as in the first prior-art example is required, so the apparatus similarly becomes high in cost.